Tue. Jan 18th, 2022

Five thousand known worlds. That is the next, most ballyhooed milestone in the ongoing hunt for exoplanets, the confirmed total of which presently tallies just a few hundred shy in our catalogues. More remarkable than these sheer numbers, however, is the diversity they reveal. A fraction of the worlds overflowing astronomers’ coffers resemble those orbiting our own sun, but most are far more alien: scorched gas giants that circle their star every few days, Neptune-sized puffballs with the density of cotton candy, and hordes of small planets packed like sardines around tiny, cool stars. Compared to such things, our own familiar and supposedly typical solar system turns out to be the oddball.

The latest bizarre exoplanet to challenge our preconceptions and reinforce just how much we still have to discover is GJ 367b, a world so strange it seems more suited for a heavy-metal album cover or the pages of a pulpy sci-fi story rather than reality. Announced December 2 in the journal Science, this planet may essentially be a glowing orb of half-molten iron three-quarters the size of Earth.

Discovered by Kristine Lam from the German Aerospace Center (DLR) and colleagues using NASA’s Transiting Exoplanet Survey Satellite (TESS), GJ 367b is a peculiar “sub-Earth” world located relatively close by, around a small red dwarf star 31 light-years away from us. TESS’s measurements showed the planet to be 9,000 kilometers wide—about a third wider than Mars—and subsequent observations using another facility, the European Southern Observatory’s High Accuracy Radial Velocity Planet Searcher (HARPS), revealed it to be just half the mass of Earth. Taken together, these results imply an astonishing density—about eight grams per cubic centimeter, close to that of pure iron. “The planet is most likely to contain about 80 percent iron by radius,” Lam says, with the rest of the planet encased by a rocky silicate mantle, a similar structure to Mercury in our solar system.

But unlike Mercury, which revolves 58 million kilometers from our sun in an 88-day orbit, GJ 367b is far closer to its star, completing an orbit in just 7.7 hours at a distance of only a million kilometers. That means the temperature of the planet’s starlight-bathed surface could be as high as 1,500 degrees Celsius, enough to melt rock and metal alike. “It’s probably not very pleasant to live on,” Lam says.

About 100 of these so-called ultrashort-period rocky exoplanets have been previously found, but GJ 367b stands out among them as the smallest and least massive ever seen. Its proximity to its star means it is most likely tidally locked by gravitational effects, meaning it always presents the same hemisphere towards the star, much like the moon does to Earth. The enormous dayside temperatures may mean this half of the planet is covered in a magma ocean. “At those temperatures you expect your silicates to be in the liquid phase,” says Alexandre Santerne from the Aix-Marseille University in France, who was not involved in this work but previously discovered another Mercury-like exoplanet. “It would be like a big magma pool.” The nightside of the planet, meanwhile, would have vastly lower temperatures, meaning it “should be solid rock,” Santerne says. At the terminator between night and day, you would expect “some transition between very cool rocks and the magma,” he says. That difference could result in tempestuous winds if the planet has any semblance of an atmosphere, but most experts believe GJ 367b’s extreme stellar proximity long ago rendered it airless.

How the planet reached its dismal state is a bit of a mystery that may carry important implications for our own solar system. The same gravitational forces that led to GJ 367b being tidally locked should have long ago disrupted the process of planet formation in the first place; planets are not thought to form extremely close to their stars. Instead, they probably migrate inward from farther out—a process that can sometimes lead to spectacular interplanetary smash-ups when worlds literally collide. Similar giant impacts may have shaped our own Mercury, which perhaps was once somewhat more Earth-like in structure. “The best story, which is not a great story, is that some object smashed into Mercury, and left behind a mostly iron object,” says Joshua Winn from Princeton University, a co-author on the GJ 367b discovery paper. But this hypothesis is “a little uncomfortable, because it invokes this collision for which we have no other evidence,” he adds. “If we figure out why these iron-rich ultrashort-period planets exist, maybe there would be some connection to the story of Mercury.”

One possibility is that, rather than being the result of a cataclysmic collision, ultrashort-period rocky worlds such as GJ 367b could be the remnant iron cores left behind when stellar effects cook off the gassy envelopes of migrating giant planets. Astronomers’ ever-expanding exoplanetary census have found both giant “hot Jupiters” as well as GJ 367b–like worlds in very close orbits around stars. Yet notably absent from these extreme environs are Neptune-like worlds midway in size between the two. The reason could be that these worlds, pushed inwards by another planet in the system, are then stripped of their hydrogen and helium atmospheres as they approach their stars, leaving only their rocky interiors behind. “It’s quite conceivable [GJ 367b] was a bigger planet that has actually been fried away,” says Lam’s former professor Don Pollacco from the University of Warwick in the U.K., who was not involved in the study. “You could imagine we’re looking at the compressed core of an evaporated planet.”

For Mercury, given its comparably greater distance from the sun, such an exotic origin story is unlikely. But further studies of Mercury, along with more observations and discoveries of ultrashort-period planets using next-generation facilities such as the James Webb Space Telescope, could get us closer to an answer of how such worlds come to be. More than anything, such work continues to highlight that, among the thousands of planets now known beyond our solar system, we continue to find strange and wonderful places. “We went looking for solar systems,” Pollacco says. What we found instead, and continue to find, were worlds unlike anything we could have imagined.

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